The Analogous Structure of Accretion Flows in Supermassive and Stellar Mass Black Holes: New Insights from Faded Changing-look Quasars

Despite their factor of ∼10⁸ difference in black hole mass, several lines of evidence suggest possible similarities between black hole accretion flows in active galactic nuclei (AGN) and Galactic X-ray binaries. However, it is still unclear whether the geometry of the disk-corona system in X-ray binaries directly scales up to AGN and whether this analogy still holds in different accretion states. We test this AGN/X-ray binary analogy by comparing the observed correlations between the UV-to-X-ray spectral index (α _OX) and Eddington ratio in AGN to those predicted from observations of X-ray binary outbursts. This approach probes the geometry of their disk-corona systems as they transition between different accretion states. We use new Chandra X-ray and ground-based rest-UV observations of faded “changing-look” quasars to extend this comparison to lower Eddington ratios of <10^-2, where observations of X-ray binaries predict a softening of α _OX in AGN. We find that the observed correlations between the α _OX and Eddington ratio of AGN displays a remarkable similarity to accretion state transitions in prototypical X-ray binary outbursts, including an inversion of this correlation at a critical Eddington ratio of ∼10^-2. Our results suggest that the structures of black hole accretion flows directly scale across a factor of ∼10⁸ in black hole mass and across different accretion states, enabling us to apply theoretical models of X-ray binaries to explain AGN phenomenology.